In principle, integrated circuits are constructed in such a way that the actual semiconductor components are arranged in a lower plane, the so-called active plane. The wiring of the semiconductor components is implemented in planes lying further above, the so-called metal planes. Depending on the complexity of the circuit, a plurality of metal planes is required in order to carry out a complete wiring. The individual metal planes are electrically isolated from one another by an insulation line. In general, attempts are made to keep the number of metal planes as low as possible, since each additional metal plane leads to a considerable increase in costs in the production of the integrated circuit.
Further requirements are made of integrated circuits which comprise security-critical circuit components. These relate to the repulse of attacks an the integrated circuit, the aim of these attacks being to covertly discover the internal processes in the security-critical circuit components or the construction thereof and thus to obtain the opportunities for manipulation or for unauthorized operations. Such attacks are known as probing, forcing, FIB, etc.
In especially security-critical cases, the affected regions are covered with an active shield and, if appropriate, an additional metal plane is provided for this in the case of an active shield, regions of a circuit are covered with a multiplicity of additional lines for which voltage and/or current flow are monitored in order to be able to detect a physical attack. A particularly critical application is present for example in an integrated circuit for DES implementation. In this application, the keys used are stored, in plain text. If an attacker succeeds in physically accessing the security-critical circuit components, the attacker can read out the keys and use them improperly.
An additional property of an integrated circuit for DES encryption that makes the construction more difficult lies in the fact that DES is very wiring-intensive owing to the many permutations. Therefore, a plurality of metal planes have to be provided anyway in order to avoid power and timing problems which result if it is attempted to dispense with a metal plane. Moreover, this would lead to enlargement of the area. The function of an active shield is nevertheless indispensable in order to ensure the security that is necessary for the application.